Ralph de Laer Kronig

Ralph Kronig

Ralph Kronig was a German-American physicist (1904-1995). He is noted for the discovery of particle spin and for his theory of x-ray absorption spectroscopy. His theories include the Kronig-Penney model for allowed and forbidden bands in solids Ralph Kronig (later Ralph de Laer Kronig) was born in 1904 from American parents in Dresden, Germany where he received his primary education. He went to New York to study at Columbia University until 1925 where he was an assistant professor.

In 1925, when Kronig was a young Columbia University PhD who had spent two years studying in Europe, he first proposed electron "spin" in Copenhagen. Werner Heisenberg and Wolfgang Pauli immediately hated the idea. They had just ruled out all imaginable actions from quantum mechanics. Now Kronig was proposing to set the electron rotating in space. Pauli especially ridiculed the idea of spin, saying that "it is indeed very clever but of course has nothing to do with reality". Faced with such criticism, Kronig decided not to publish his theory and the idea of electron spin had to wait for others to take the credit. Ralph Kronig, had come up with the idea of electron spin several months before Uhlenbeck and Goudsmit, however, most textbooks credit these two men with the discovery.

Ralph Kronig did not hold a grudge against Pauli for this turn of events. In fact, Kronig and Pauli remained friends for many years into the future. They exchanged many ideas in physics through letters. However, many have argued (especially Dutch physicists) that Pauli only received the Nobel Prize for his exclusion principle due to Kronig's theory of particle spin. The argument stems from the fact that Kronig had told Pauli about electron spin before Pauli had published his paper showing that two electrons can inhabit the same orbital (W. Pauli, “On the Connexion between the Completion of Electron Groups in an Atom with the Complex Structure of Spectra”, Z. Physik 31, 765ff, 1925). Months later when Uhlenbeck and Goudsmit came up with particle spin, it seemed to verify Pauli's paper.

In 1927, Kronig returned to Europe and remained there working in different prominent centres of research: Copenhagen, London, Zürich (where for a year he was Pauli's assistant). Around 1930 he settled in the Netherlands: first in Utrecht, afterwards in Groningen. Kronig and Penney (1931) published a one dimensional model of a crystal that showed how the electrons in a crystal would be dispersed into allowed and forbidden bands by scattering from the extended linear array of atoms. Kronig was later in 1939 to be appointed a professor of theoretical physics in the Netherlands and to devote his life to research. The Max Planck medal was awarded to Ralph Kronig in 1962.